The present invention relates to a polymeric reference electrode membrane, which comprises (a) one selected from a porous polymer or a hydrophilic plasticizer; (b) a lipophilic polymer; and optionally an adhesion-enhancing material, and a conventional-type reference electrode and a solid-state reference electrode equipped with the polymeric reference electrode membrane. Further, the present invention is concerned with a miniaturized multi-potentiometric sensor comprising a solid-state reference electrode and a set of ion-selective electrode.
Potentiometry, as an electroanalytical chemistry, has been widely used in the determination of quantities of electrolytes in liquid samples in the fields of clinical, environmental, food and industrial analysis. However, when electrochemical analysis is performed in the laboratory, the sample may be contaminated or denatured due to the delay in sample transportation. In order to circumvent the problems, it is preferred that samples be analyzed at the spot. Particularly, medical analyzers that analyze clinical samples such as blood are necessary to use samples of as little volume as possible in order to minimize the patient""s discomfort upon sampling, in addition to overcoming the analysis delay problems. Additionally, low prices are required for analyzers such that medical tests performed therewith can be universalized. In consequence, analyzers should be minimal in size, portable and inexpensive.
To minimize electrochemical analyzers is essential to reduce the dimensions of electrode systems. Generally, electrode systems are composed of a reference electrode and a working electrode. Compared with other components of electrode systems, more effort has gone into the miniaturization of a working electrode, so that practically various structures of practical working electrode have been developed. Meanwhile, the reference electrode is not advanced in size reduction, so that its large sizes make it difference to miniaturize electrochemical analyzer in miniaturizing
Typically, electrochemical sensors have two-electrode system provided with working electrodes, which sensitively respond to species of interest, and reference electrodes, which maintain constant potentials without responding to the species. The working electrodes measure not values of absolute potentials, but values relative to the constant potential of the reference electrodes, that is, potential differences.
In electrochemical analysis, therefore, reference electrodes must maintain constant potentials irrespective of surrounding conditions and return instantaneously to initial equilibrium potentials after a current flow. Reference electrodes containing insoluble metal salts such as Ag/AgCl, are not dissolved in electrolytes. Another requirement for a reference electrode should be a short preconditioning time thereby being available to achieve point-of-care. Of course, high reproducibility is essential for making reference electrodes reliable.
As an effort to develop a planar-type small solid-state reference electrode suitable for use in electrochemical measurement, a reference electrode is reported that a layer of potassium chloride-dissolved glass paste having low melting point or a layer of thermosetting silicone polymer paste is formed on a silver/silver chloride layer coated through a screen printing method, and then a hydrophobic polymer insulating paste is overlaid onto the paste layer. (Cranny, A. W. J. and Atkinson, J. K. Meas. Sci. Technol. 1998, 9, 1557-1565). When the polymer insulating paste is overlaid, small hydration holes should be in the insulating paste to allow the diffusion of potassium chloride-dissolved paste layer to aqueous solution. Such reference electrode has long use lifetime, but it takes about 1 hour or more for preconditioning the electrode.
As an example of another type of solid-state reference electrode, it is reported that potential difference is offset between an anion-selective electrode and a cation-selective electrode (Nagy, K; Eine, K; Syverud, K; Aune, O. J. Electrochem. Soc. 1997, 144, L1-L2). In the reference electrode structure, the anode-selective membrane comprises a support, a plasticizer, an anionic lipophilic additive, and an anion exchange membrane, while the cation-selective membrane comprises a support, a plasticizer, a cationic lipophilic additive, and a cation exchange membrane.
The reference electrode requires long preconditioning time, and its use may be restricted until an anion-selective electrode and a cation-selective electrode are simultaneously activated, because the method needs to offset the potential difference between them. The reference electrode can offset the sensing of cations and anions having the same charge number, but cannot function owing to a potential difference attributable to the sensing difference between ion-selective membrane electrodes for ion species having different charge number and mixed ion species.
Another example of a small solid-state reference electrode, ultramicroelectrodes have been proposed, suitable for square wave anodic stripping voltammetry using an anion exchange membrane, Nafion, or polymers such as polyurethane (Melissa A. Nolan; Sandie H. Tan; Samuel P. Kounaves. Anal. Chem. 1997, 69, 1244-1247).
The ultramicroelectrode is fabricated that silver/silver chloride is immersed in sodium chloride (NaCl)/poly (vinyl chloride) solution, and coated with salt, followed by coating with Nafion or polyurethane. In square wave anodic stripping voltammetry, chloride ions present in samples react with metal ions to form a complex, so it is important to block inflow of chloride ions. Nafion introduced to the reference electrode acts as an anion exchange membrane and thus is responsible for preventing the chloride ion from being introduced into the samples. However, Nafion used for the anion exchange membrane of the reference electrode suffers from poor adhesion, long preconditioning time, noise, and too high sensitivity to ions, and thus cannot be applied to potentiometric measurement system.
In addition, a reference field effect transistor based on perchlorate, and fluoride ion-selective field effect transistor, as other small solid-state reference electrode, is reported (Potter W.; Dumschat, C.; Cammann K. Anal. Chem. 1995, 67, 4586-4588). The reference field effect transistor based on perchlorate ion-selective field effect transistor can be easily miniaturized, but cannot be applied to voltammetry because of the high resistance of the reference field effect transistor itself and sensing to electrolyte of high concentration. Also, instability of the ion-selective field effect transistor itself causes necessarily to a flow potential of the reference field effect transistor, which is a reference electrode of the ion-selective field effect transistor.
Meanwhile, the reference field effect transistor based on fluoride ion-selective field effect transistor containing a polymeric membrane capable of reducing a diffusion of calcium fluoride, its inner electrolyte, is affected by not only concentrations of fluoride ions and calcium ions in the samples but also interfacial contact potential between the inner electrolyte and the sample, due to low solubility of calcium fluoride.
Leading to the present invention, the intensive and thorough research on polymeric reference electrode membranes, conducted by the present inventors aiming to overcome the problems encountered in prior arts, resulted in the finding that, when a polymeric reference electrode membrane comprises (a) one selected from a porous polymer or a hydrophilic plasticizer; (b) a lipophilic polymer; and optionally an adhesion-enhancing material, the membrane can allow short preconditioning time required for sample analysis whereby improving reproducibility and yield due to excellent adhesion of the membrane to solid substrate, and maintain stable potential for mixed ion species and protein-containing calibration solutions, serum and whole bloods, thereby resulting in a miniaturized multi-potentiometric sensor having a solid-state reference electrode equipped with the polymeric reference electrode membrane.
Therefore, it is an object of the present invention to provide a polymeric reference electrode membrane comprising (a) one selected from a porous polymer or a hydrophilic plasticizer; (b) a lipophilic polymer; and optionally an adhesion-enhancing material.
It is another object of the present invention to provide a conventional-type reference electrode equipped with the polymeric reference electrode membrane.
It is further object of the present invention to provide a solid-state reference electrode, which characterizes a double layered or a mono-layered solid-state reference electrode, equipped with the polymeric reference electrode membrane.
It is a still further object of the present invention to provide a multi-potentiometric sensor comprising a reference electrode selected from a double layered or a mono-layered solid-state reference electrode and a set of ion-selective electrode.